American and European anemones. Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria. They are named for the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Hexacorallia. Anthozoa often have large polyps that allow for digestion of larger prey and also lack a medusa stage. Many sea anemones form symbiotic relationships with single-celled algae, whether zooxanthellae or zoochlorellae, that live within their cells. The global trade in marine ornamentals for aquariums is rapidly expanding, and threatens sea anemone populations as the trade depends on collection from the wild, and the animals grow and reproduce relatively slowly. 49th plate from Ernst Haeckel's Kunstformen der Natur (Art forms of Nature) of 1904.